Ultrasonic flow measuring devices are often applied in process and automation technology. They permit volume flow and/or mass flow in a pipeline to be determined in a simple manner.
Known ultrasonic flow measuring devices frequently work according to the Doppler principle or according to the travel time difference principle. In the travel time difference principle, the different travel times of ultrasonic pulses are evaluated relative to the flow direction of the liquid. For this, ultrasonic pulses are sent at a particular angle to the tube axis both in the direction of the flow as well as also counter to the flow. From the travel-time difference, the flow velocity, and therewith, in the case of a known diameter of the pipeline section, the volume flow, can be determined.
In the case of the Doppler principle, ultrasonic waves with a particular frequency are coupled into the liquid, and the ultrasonic waves reflected by the liquid are evaluated. From the frequency shift between the in-coupled and reflected waves, the flow velocity of the liquid can likewise be determined.
Reflections in the liquid occur, however, only when small air bubbles or impurities are present, so that this principle is mainly put to use in the case of contaminated liquids.
The ultrasonic waves are produced and received with the assistance of so-called ultrasonic transducers. These are fixedly placed on the tube wall of the relevant pipeline section. More recently, clamp-on ultrasonic flow measuring systems are also obtainable. In the case of these systems, the ultrasonic transducers are only pressed onto the tube wall with a clamp. Such systems are known, for example, from EP 686 255 B1, U.S. Pat. Nos. 4,484,478 or 4,598,593. A further ultrasound flow measuring device, which works according to the travel time difference principle, is known from U.S. Pat. No. 5,052,230. The travel time is, in this case, ascertained by means of short ultrasonic pulses.
A great advantage of clamp-on ultrasound flow measuring systems is that they do not contact the measured medium and they can be placed on an already existing pipeline. Disadvantageous is a high effort needed when mounting the clamp-on systems to orient the individual ultrasonic transducers with respect to each other, such depending on many parameters, e.g. tube wall thickness, tube diameter, and velocity of sound in the measured medium.
The ultrasonic transducers normally comprise an electromechanical transducer, in industrial process measurements technology, most often a piezoceramic; and a coupling layer, also called a coupling wedge or, less frequently, a lead-in element. The coupling layer is, in such case, most often manufactured from synthetic material, e.g. plastic. In the electromechanical transducer element, the ultrasonic waves are produced, and led via the coupling layer to the tube wall, and from there, guided into the liquid. Since the velocities of sound in liquids and synthetic materials are different, the ultrasonic waves are refracted during the transition from one medium to another. The angle of refraction is determined in a first approximation according to Snell's law. The angle of refraction is thus dependent on the ratio of the propagation velocities in the media.
Between the piezoelectric element and the coupling layer, another coupling layer can be arranged, a so-called adapting or matching layer. The adapting or matching layer performs, in such case, the function of transmission of the ultrasonic signal, and simultaneously the reduction of reflection off interfaces between two materials caused by different acoustic impedances.
U.S. Pat. No. 6,397,683 B1 shows two clamp-on ultrasonic sensors which are securable on a pipeline by means of two affixing bands. For adjusting the spacing of the two sensors, at least one sensor of the pipeline is to be loosened. Its affixing band is likewise to be loosened. Both of these are to be fixed anew after a new orienting. The coupling element of one of the ultrasonic sensors, together with the piezoelectric ultrasound transducer element placed on it, is seated resiliently in a housing.
U.S. Pat. No. 5,131,278 shows a possible embodiment for shiftably seating two ultrasonic transducers on a track, which is mountable fixedly on the measuring tube. A track for the shiftable securement of ultrasonic sensors is likewise presented in EP 1 396 707 A1 and EP 1 840 528 A2. The sensors are, in such case, mounted resiliently in the track.
Described in EP 1 783 464 A2 is a guide frame as sensor holding system, which is securable on the measuring tube by means of two securement systems. A housing is placed over this for protection of the sensors. For maintenance and for shifting the sensors, the housing must be removed.
All these documents share the common characteristic that the sensors are shiftable during measurement operation. The holding apparatuses are, in such case, embodied in such a manner, that, after their securement on the measuring tube, the sensors are shiftable into the desired position. In order to protect the sensors from mechanical influences from outside, protective housings, in part to be applied separately, are provided.